In a reciprocating engine such as an automobile engine, the exhaust valve is generally driven by a cam that rotates in synchronism with the crankshaft.
In this case, if the valve is opened suddenly in an effort to maintain the exhaust flowrate, there is a rapid expulsion or blowdown of combustion gases from the engine so that exhaust pulsation noise increases.
Tokkai Hei 1-159417 published by the Japanese Patent Office in 1989, for example, discloses an arrangement for an engine having two exhaust valves in each cylinder, and by changing the shape of the cams driving the exhaust valves, these valves are made to open at different timings so that blowdown occurs more gradually for the cylinders as a whole.
However, as a cam has to be provided for each exhaust valve, the construction of the valve apparatus becomes more complex and costly.
This system moreover cannot be applied to an engine having only one exhaust valve.